In catalytic cracking reaction systems, it is important to maximize contact of hydrocarbon with the catalytic cracking catalyst in order to maximize conversion. However, if contact time is excessive, there will be over-cracking of the hydrocarbon with excessive deposition of carbonaceous material, i.e., coke adheres to the catalyst. If there is excessive coke buildup, regeneration and reuse of the catalyst becomes very difficult.
One method of reducing contact time in a catalytic cracking system is to reduce the cross-sectional area of the system in which the reaction occurs. This reduction in area has been accomplished in closed cyclone systems. In such systems, cyclones are placed within a reaction vessel, and are positioned so that the cyclone inlets maintain direct contact with the hydrocarbon and catalyst mixture.
U.S. Pat. No. 4,043,899 discloses a catalytic cracking system which incorporates a closed cyclone system. In closed cyclone systems like that shown in U.S. Pat. No. 4,043,899, typically about 90% or more of the hydrocarbon flow goes directly from the riser outlet, through the internal reactor cyclones, and to the plenum chamber without going into the reactor vessel itself. Stripout vapors from the stripping section, and any other vapors entrained in the reactor vessel, generally enter the plenum chamber though ports or holes in the cyclone system or there are additional cyclones which are specifically used to capture the entrained vapors.
U.S. Pat. Nos. 4,588,558 and 4,749,471 disclose a closed cyclone system within a catalytic cracking reaction vessel. The closed system shown in these patents incorporates an annulus separating the riser from the collection conduit of the cyclones. The riser and the collection conduit maintain essentially constant dimensions throughout the reaction vessel.
A problem inherent in catalytic cracking systems which incorporate the closed cyclone designs of the prior art is that the hydrocarbon feed and catalytic cracking catalyst are not intensely mixed, resulting in poor contact between hydrocarbon and catalyst. The closed cyclone designs of the prior art are also problematic in that vapors can accumulate in the dilute phase of the reactor vessel, where they remain for an extended period of time. This adversely affects the stripping of hydrocarbon entrained in the catalyst, product yield and overall product quality. It is, therefore, an object of this invention to overcome many of the problems inherent in known catalytic cracking systems.